A computer network is comprised essentially of a group of devices that are coupled together by a communication medium. A network may be as simple as two devices that are wired to each other, or as complex as the Internet. The devices in a network can communicate with each other and provide services to each other (or, conversely, use each other's services).
The devices may be of many different types. For example, some may be host devices (hosts) that initiate commands, while others may be devices which are designed to service the hosts (e.g., data storage devices). Networks may be configured in many different ways. For instance, referring to FIG. 1, one particular configuration of a storage area network (SAN) is illustrated. It should be noted that a SAN may be configured in many different ways, and that the configuration illustrated in FIG. 1 is exemplary.
In the SAN of FIG. 1, a plurality of hosts 11-13 are coupled to a sequential device 15 by a transmission medium 14. (“Sequential device,” as used here, refers to a device that is sequentially accessed, such as a tape drive, rather than one which can be randomly accessed, such as a disk drive.) Sequential device 15 is configured to sequentially process requests for data which are received from hosts 11-13.
Hosts 11-13 may transmit several different kinds of commands to sequential device 15. One type of command is an inquiry command. “Inquiry command,” for the purposes of this disclosure, will refer to commands which request data that, for the most part, is static. This data, which will be referred to herein as “inquiry data,” typically relates to the device itself, rather than data which is designed to store or generate. For instance, inquiry data may include the serial number, manufacturer, configuration, version number, or similar data. It should be noted that, however, that inquiry data may also include data that changes relatively infrequently, rather than being strictly static.
Inquiry commands may be submitted to device 15 for various reasons. For example, when a new host is booted, it checks to see what other devices are connected to the network. Hosts may also periodically use inquiry commands to obtain information regarding the availability of devices on the network. If a device responds to the command, the host will receive the responsive inquiry data and will be aware that the device is available. If the device does not respond, the host may assume either that the device is no longer connected to the network, or that the device is no longer functioning properly.
One of the problems with handling inquiry commands in sequential devices is that these commands may be submitted to a sequential device just after the device has received a command that will take a substantial amount of time to process. Conventionally, no distinction is made between inquiry commands and other commands, so the inquiry command in this example will be handled in the same manner as any other command it will be serviced when the device becomes available. Until the previously received command is serviced, the inquiry command will be blocked. If the inquiry command is blocked for long enough, it will time out. As a result of having timed out, the host may assume that the device is no longer operational or no longer connected to the network, even though it is both connected and operational.
It would therefore be desirable to provide a means for servicing inquiry commands in a sequential device while other commands are being processed by the device.